1. Field of the Invention
The present invention relates to a method for producing a printing form by means of scanning an original and the recording surfaces in accord with a scanning or, respectively, conforming recording raster upon employment of a random generator for the distribution of the smallest points which are still printable to each raster field of the recording surface, whereby the plurality of said points which are still printable determines the respective tonal value.
2. Description of the Prior Art
Autotypical tonal value analysis is now executed with the assistance of regular rasters to a large degree. It is only for special purposes, for example, for facsimile printing, that high frequency rasters, i.e., rasters with a high spatial frequency of the raster points or, respectively, image information are still employed with an irregular structure (for example, German OS No. 19 21 642; German OS No. 16 22 340). The raster constant is usually selected in such manner that, upon consideration of the printability properties of the printing materials and of the density scale, optimum reproduction properties ensue. The statistical distribution of the raster points is undertaken with the assistance of a random generator.
Predominantly employed contact screens and intersecting line screens are the cause of disadvantageous density skips which occur when the points exposed on begin to contact one another. This effect can be reduced by means of changing the shape of the point, for example, chain point form.
In multicolor printing, partial images are printed on top of one another. Thereby, upon employment of regular rasters, raster points come to lie next to one another or on top of one another in a regular interchange, so that regular patterns become perceptible. Their size is determined by the raster constants and by the angulation difference of the rasters employed. They are called Moire. Even given optimum angulation differences as described, for example, in the German LP No. 20 12 728 (corresponds to U.S. Pat. No. 4,084,183), a residual Moire, so-called rosettes remain. Particularly given half-wise surface coverage and the standard raster constants which take the printability properties into consideration, the residual Moire can still be easily seen from a normal observation distance and is therefore considered to be disturbing.
Further, a method has been specified in the trade journal "RCA Review", September 1970, Vol. 31, No. 3, pages 517 through 533, in which, with the assistance of a random generator, the surface of the raster field is composed of a different number--said number corresponding to the tonal value--of identical points of the size of the smallest printable point. In terms of surface, the smallest printable point covers approximately 4 through 8% of the standard raster field size, i.e., a maximum of 25 such smallest printable points to be exposed can be accommodated on this surface. In turn deriving therefrom is the fact that only 25 stages of tonal value are possible. This, however, is not sufficient for a high-grade reproduction, since skips in tonal value occur within the reproduced image at the boundaries of tonal values. A further disadvantage of this method consists therein that, given a high tonal value, the smallest surface remaining free within the raster field cannot be kept large enough so that printing ink flows in it or bleeds into it, which leads to a further reduction of the exploitable tonal value steps.
Overall, it can be stated that, upon employment of high frequency rasters, i.e., rasters with a high spatial frequency with an irregular structure, no Moire occurs since these rasters themselves are not periodic, but that such rasters have previously hardly been employed because of deficient linearity, poor reproducibility and their relatively slight density scale. They are almost exclusively employed, as already mentioned, for special purposes, for example, for facsimile printing or for graphic output in electronic data processing.